Four KHz gas discharge laser

ABSTRACT

The present invention provides an excimer laser capable of producing a high quality pulsed laser beam at pulse rates of about 4,000 Hz at pulse energies of about 5 mJ or greater. A preferred embodiment is an ArF excimer laser specifically designed as a light source for integrated circuit lithography. An improved wavemeter with special software monitors output beam parameters and controls a very fast PZT driven tuning mirror and the pulse power charging voltage to maintain wavelength and pulse energy within desired limits. In a preferred embodiment two fan motors drive a single tangential fan which provides sufficient gas flow to clear discharge debris from the discharge region during the approximately 0.25 milliseconds between pulses.

[0001] The present invention is a continuation-in-part of Ser. No.09/834,840, filed Apr. 13, 2001, Ser. No. 09/794,782, filed Feb. 27,2001, Ser. No. 09/771,789, filed Jan. 29, 2001, Ser. No. 09/768,753,filed Jan. 23, 2001, Ser. No. 09/684,629, filed Oct. 6, 2000, Ser. No.09/597,812, filed Jun. 19, 2000 and Ser. No. 09/473,852, filed Dec. 27,1999. This invention relates to gas discharge lasers and in particularto high repetition rate gas discharge lasers.

BACKGROUND OF THE INVENTION Electric Discharge Gas Lasers

[0002] Electric discharge gas lasers are well known and have beenavailable since soon after lasers were invented in the 1960s. A highvoltage discharge between two electrodes excites a gaseous gain medium.A resonance cavity containing the gain medium permits stimulatedamplification of light which is then extracted from the cavity in theform of a laser beam. Many of these electric discharge gas lasers areoperated in a pulse mode.

Excimer Lasers

[0003] Excimer lasers are a particular type of electric gas dischargelaser and have been known as such since the mid 1970s. A description ofan excimer laser, useful for integrated circuit lithography, isdescribed in U.S. Pat. No. 5,023,884 issued Jun. 11, 1991 entitled“Compact Excimer Laser.” This patent has been assigned to Applicants'employer, and the patent is hereby incorporated herein by reference. Theexcimer laser described in Patent '884 is a high repetition rate pulselaser. In FIG. 1, the principal elements of the laser 10 are shown.(FIG. 1 corresponds to FIG. 1 and FIG. 2 corresponds to FIG. 7 in Patent'884.) The discharges 22 are between two long (about 23 inches)electrodes 18 and 20 spaced apart by about ⅝ inch. Repetition rates ofprior art lasers, like the one described, are typically within the rangeof about 100 to 2000 pulses per second. These high repetition ratelasers are usually provided with a gas circulation system. In the abovereferred to laser, this is done with a long squirrel-cage type fan 46,having about 23 blades 48. The fan blade structure is slightly longerthan the electrodes 18 and 20 and provides sufficient circulation sothat at pulse operating rates, the discharge disturbed gas between theelectrodes is cleared between pulses. The shaft 130 of fan 46 issupported by two ball bearings 132 as shown in FIG. 2A which is FIG. 9of Patent '884. The gas used in the laser contains fluorine which isextremely reactive. The fan rotor driving fan shaft 130 is sealed,within the same environmental system provided by housing structuremembers 12 and 14, by sealing member 136 as explained at column 9, line45 of Patent '884, and the motor stator 140 is outside sealing member136 and thus protected from the corrosive action of the fluorine gas.However, bearing 132 is subjected to the corrosive action of the chambergas as is the lubrication used in the bearing. Corrosion of the bearingsand bearing lubrication can contaminate the gas.

Modular Design

[0004] These excimer lasers, when used for integrated circuitlithography, are typically operated on a fabrication line“around-the-clock”; therefore down time can be expensive. For thisreason most of the components are organized into modules which can bereplaced normally within a few minutes.

Line Narrowing

[0005] Excimer lasers used for lithography must have its output beamreduced in bandwidth to a fraction of a picometer. This “line-narrowing”is typically accomplished in a line narrowing module (called a “linenarrowing package” or “LNP”) which forms the back of the laser'sresonant cavity. This LNP typically is comprised of delicate opticalelements including prisms, a mirror and a grating. As repetition ratesincrease maintaining stable performance by the LNP becomes a seriouschallenge.

Pulse Power

[0006] Electric discharge gas lasers of the type described in U.S. Pat.No. 5,023,884 utilize an electric pulse power system such as thatdescribed in FIG. 3 to produce the electrical discharges, between thetwo electrodes. In such prior art systems, a direct current power supply22 charges a capacitor bank called “the charging capacitor” or “C₀” 42to a predetermined and controlled voltage called the “charging voltage”for each pulse. The magnitude of this charging voltage may be in therange of about 500 to 1000 volts. After C₀ has been charged to thepredetermined voltage, a solid state switch 46 is closed allowing theelectrical energy stored on C₀ to ring very quickly through a series ofmagnetic compression circuits comprising capacitor banks 52, 62 and 82and inductors 48, 54 and 64 and a voltage transformer 56 to produce highvoltage electrical potential in the range of about 16,000 volts acrossthe electrode which produces the discharge which lasts about 50 ns.

[0007] In prior art systems on the market the time between the closingof the solid state switch and the discharge is in the range of about 5microseconds; however, the charging of C₀ accurately to the pre-selectedvoltage has in the past required about 400 microseconds which was quickenough for pulse repetition rates of less than about 2,000 Hz. Thereader should understand that accurate charging of C₀ is very importantsince the control of the voltage level on C₀ is in these systems theonly practical control the laser operator has on the discharge voltagewhich in turn is the primary determiner of laser pulse energy.

Heat Exchanger

[0008] Prior art excimer lasers used for integrated circuit lithographytypically require a system for cooling the laser gas which is heatedboth by the electric discharges and by the energy input throughcirculating fan discussed above. This is typically done with a watercooled, finned heat exchanger shown at 58 in FIG. 1. A doubling or moreof the repetition rate of a laser more than doubles the heat generatedin the laser primarily because power required to circulate the laser gasincreases as the cube of the required gas velocity.

Control of Beam Quality

[0009] When used as a light source for integrated circuit lithography,the laser beam parameters (i.e., pulse energy, wavelength and bandwidth)typically are controlled to within very tight specifications. Thisrequires pulse-to-pulse feedback control of pulse energy and somewhatslower feedback control of wavelength of the line narrowed output beam.A doubling or more of the pulse rate requires these feedback controlsystems to perform much faster.

[0010] What is needed is a better laser design for a pulse gas dischargelaser for operation at repetition rates in the range of about 4,000pulses per second.

SUMMARY OF THE INVENTION

[0011] The present invention provides an excimer laser capable ofproducing a high quality pulsed laser beam at pulse rates of about 4,000Hz at pulse energies of about 5 mJ or greater. A preferred embodiment isan ArF excimer laser specifically designed as a light source forintegrated circuit lithography. An improved wavemeter with specialsoftware monitors output beam parameters and controls a very fast PZTdriven tuning mirror and the pulse power charging voltage to maintainwavelength and pulse energy within desired limits. In a preferredembodiment two fan motors drive a single tangential fan which providessufficient gas flow to clear discharge debris from the discharge regionduring the approximately 0.25 milliseconds between pulses.

BRIEF DESCRIPTION OF THE DRAWINGS

[0012]FIGS. 1, 2 and 2A show features of a prior art laser system.

[0013]FIG. 3 is a circuit diagram of a prior art pulse power system.

[0014]FIG. 4A shows a front view of a preferred embodiment of thepresent invention.

[0015]FIG. 4B shows a cross section of a laser chamber of the preferredembodiment.

[0016]FIG. 5 shows an electrical circuit diagram of a preferred pulsepower system.

[0017]FIGS. 6A and 6B show two preferred resonant power supplies.

[0018]FIGS. 7, 7A and 8 show techniques for cooling pulse powercomponents.

[0019]FIGS. 9A and 9B show views of a saturable inductor.

[0020]FIG. 10 shows a pulse transformer.

[0021]FIG. 10A shows a pulse transformer core.

[0022]FIG. 11 shows a technique for cooling a first saturable inductor.

[0023]FIGS. 12, 12A and 12B show a technique for cooling a secondsaturable inductor.

[0024]FIGS. 13 and 13A show views of the discharge region in the chamberof a preferred embodiment.

[0025]FIG. 14 shows the logout of a preferred embodiment.

[0026]FIGS. 14A, 14B, 14C and 14D are charts and graphs explaining thecalculation of wavelengths and bandwidths.

[0027]FIGS. 14E, 14F, 14G and 14H show view of an etalon used forwavelength and bandwidth monitoring.

[0028]FIG. 15 is a block diagram showing components used for wavelengthand bandwidth calculation.

[0029]FIG. 16 is a block diagram showing features of the laser systemused for controlling the wavelength and pulse energy of the laser beam.

[0030] FIGS. 16A, 16B1 and 16B2 are drawings showing techniques forcontrolling the LNP tuning mirror.

[0031]FIG. 16C shows the effect of piezoelectric control of the tuningmirror.

[0032]FIGS. 16D and 16E are diagrams showing wavelength controlalgorithms.

[0033]FIGS. 17, 17A, 17B and 17C show techniques for purging a gratingface.

[0034]FIG. 18 shows a two-motor blower control system.

[0035]FIG. 18A shows a blower blade structure.

[0036]FIGS. 19, 19A and 19B shows features of a preferred N₂ purgesystem.

[0037]FIGS. 20, 20A and 20B show features of a preferred shutter.

[0038]FIGS. 21 and 21A show features of preferred water cooled finnedheat exchanges.

[0039]FIGS. 22A, 22B, 22C and 22D show how the chamber is rolled intoposition in the laser cabinet.

[0040]FIGS. 22E, 22F, 22G and 22H show features of a first bellowsdesign.

[0041]FIGS. 22I, 22J, 22K and 22L show features of a second bellowsdesign.

[0042]FIG. 22M show features of a purge technique for purging high UVflux regions of the output beam train.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS First Preferred Embodiment

[0043] Described below is a first preferred embodiment of the presentinvention. It is an argon-fluoride (ArF) excimer laser designed toproduce 5 mJ narrow-band approximately 193.368 nanometer (193,368 pm)pulses at pulse rates of up to about 4 KHz. Specifications for theselasers include a bandwidth specification range of less than 0.35 pm(FWHM) and less than 0.95 pm (95% integral). Specifications also callfor 3 sigma wavelength stability of less than 0.12 pm and a 50-pulsedose stability of less than 0.3 mJ. FIG. 4A is a front view of thispreferred embodiment with the doors removed and many of the lasercomponents identified. The laser is a modular unit and designed so thatcomplete modules can be replaced very quickly and easily in order tokeep laser down time to a minimum. Going clockwise around FIG. 4A theidentified components are:

[0044] status lamp 1K indicating the operational status of the laser,

[0045] control module 2K which controls the functioning of the laserbased on input control signals from a hand held terminal (not shown) ora master controller of a lithography machine,

[0046] compression head module 3K which is part of the lasers pulsepower system and provides the final stages of pulse compression ofelectrical pulses which charge a peaking capacitor bank located on topof the laser chamber,

[0047] stabilization module 4K also called the wavemeter which monitorsthe laser pulses and provides feedback signals controlling wavelengthand pulse energy,

[0048] automatic shutter module 5K with power meter,

[0049] MFT power supply 6K providing high voltage power to a metalfluoride trap (a filter) located on the laser chamber module,

[0050] left side blower motor 7K,

[0051] laser chamber module 8K,

[0052] interface module 9K providing interface circuits to mate thelaser controls with lithography machine controls,

[0053] cooling supply module 10K,

[0054] cooling water distribution module 11K,

[0055] laser gas supply module 12K,

[0056] ventilation assembly 13K for venting the laser cabinet gases tooutside atmosphere including a smoke detector,

[0057] right side blower motor 14K,

[0058] line narrowing module 15K also called the line narrowing packageor LNP,

[0059] right side blower motor controller 16K,

[0060] left side blower motor controller 17K,

[0061] commutator module 18K which contains a Co changing capacitor bankand an electrical circuit for initiating electrical pulses and forproviding early stage pulse compression and pulse voltage amplification,

[0062] resonant charger module 19K for providing very fast resonantcharging of the Co capacitor bank,

[0063] high voltage power supply module 20K for producing high voltageDC power from standard facility three phase AC power,

[0064] AC/DC distribution module 21K.

[0065]FIG. 4B is a cross section drawing of a laser chamber 10A of afirst preferred embodiment of the present invention. The principalchamber components are housing structure members 12A and 14A, cathode18A and anode 20A downstream preionizer tube 60, peaking capacitor bank62 and electrostatic trap unit 64 (all of which are similar to the priorart corresponding components shown in FIG. 1). The chamber includes anew anode support flow shaping structure 48, a new upper flow shapingstructure 50, gas turning vanes 52, a new 5 inch diameter tangentialtype fan blade structure 46A and four water cooled heat exchanger units58A.

[0066]FIG. 16 is a block diagram showing features of the laser systemwhich are important for controlling the wavelength and pulse energy ofthe output beam.

[0067] Important improvements over prior art gas discharge lasersinclude:

[0068] 1) Improved flow path for circulating chamber laser gas

[0069] 2) Water cooled pulse power system

[0070] 3) Ultra fast wavemeter with fast control algorithm

[0071] 4) New high duty cycle LNP with combination PZT and stepper motordriven tuning mirror

[0072] 5) Large tangential fan with dual water-cooled brushless DC driveblower motors with special controllers

[0073] 6) Ultra pure nitrogen purge system for optics protection

[0074] 7) Sealed shutter with power meter

[0075] 8) Improved heat exchanger arrangement

[0076] 9) Beam seal system

LASER CHAMBER Heat Exchangers

[0077] This preferred embodiment is designed to operate at pulserepetition rates of 4,000. Clearing the discharge region of dischargeaffected gas between pulses requires a gas flow between the electrodes18A and 20A of up to about 67 m/s. To achieve these speeds, the diameterof tangential fan unit has been set at 5 inches (the length of the bladestructure is 26 inches) and the rotational speed has been increased toabout 3500 rpm. To achieve this performance the embodiment utilizes twomotors which together deliver up to about 4 kw of drive power to the fanblade structure. At a pulse rate of 4000 Hz, the discharge will addabout 12 kw of heat energy to the laser gas. To remove the heat producedby the discharge along with the heat added by the fan four separatewater cooled finned heat exchanger units 58A are provided. The motorsand the heat exchangers are described in detail below.

[0078] A preferred embodiment of the present invention utilizes fourfinned water cooled heat exchangers 58A shown generally in FIG. 4. Eachof these heat exchangers is somewhat similar to the single heatexchangers shown at 58 in FIG. 1 having however substantialimprovements.

Heat Exchanger Components

[0079] A cross sectional drawing of one of the heat exchangers is shownin FIG. 21. The middle section of the heat exchanger is cut out but bothends are shown. FIG. 21A shows an enlarged view of the end of the heatexchanger which accommodates thermal expansion and contraction.

[0080] The components of the heat exchanger includes a finned structure302 which is machined from solid copper (CU 11000) and contains twelvefins 303 per inch. Water flow is through an axial passage having a borediameter of 0.33 inch. A plastic turbulator 306 located in the axialpassage prevents stratification of water in the passage and prevents theformation of a hot boundary layer on the inside surface of the passage.A flexible flange unit 304 is a welded unit comprised of inner flange304A, bellows 304B and outer flange 304C. The heat exchanger unitincludes three c-seals 308 to seal the water flowing in the heatexchanger from the laser gas. Bellows 304B permits expansion andcontraction of the heat exchanger relative to the chamber. A double portnut 400 connects the heat exchanger passage to a standard {fraction(5/16)} inch positional elbow pipe fitting which in turn is connected toa water source. O-ring 402 provides a seal between nut 400 and finnedstructure 302.

The Turbulator

[0081] In a preferred embodiment, the turbulator is comprised of fouroff-the-shelf, long in-line mixing elements which are typically used tomix epoxy components and are available from 3M Corporation (StaticMixer, Part No. 06-D1229-00). The in-line mixers are shown at 306 inFIG. 21 and 21A. The in-line mixers force the water to flow along agenerally helical path which reverses its clockwise direction aboutevery pitch distance (which is 0.3 inch). The turbulator substantiallyimproves heat exchanger performance. Tests by Applicants have shown thatthe addition of the turbulator reduces the required water flow by afactor of roughly 5 to maintain comparable gas temperature conditions.

Flow Path

[0082] In this preferred embodiment, gas flow into and out of thedischarge region has been greatly improved over prior art laserchambers. Vane structure 66 is designed to normalize gas velocity in theregion 68, just downstream of the fan blade structure at about 20 m/s.The velocity of the gas then speeds up in the discharge region to thedesign velocity of 67 m/s. At about 4 inches downstream of the center ofthe discharge region, the flow cross section increases at an angle of 20degrees from about ⅝ inch to about 4 inches before the gas is turned byfour turning vanes 52. This permits recovery of a large percentage ofthe pressure drop through the discharge region.

Blower Motors and Large Blower

[0083] This first preferred embodiment of the present invention providesa large tangential fan driven by dual motors for circulating the lasergas. This preferred arrangement as shown in FIG. 24 provides a gas flowbetween the electrode of 67 m/sec which is enough to clear a space ofabout 1.7 cm in the discharge region between 4,000 Hz pulses.

[0084] A cross section blade structure of the fan is shown as 64A inFIG. 4. A prospective view is shown in FIG. 18A. The blade structure hasa 5 inch diameter and is machined out of a solid aluminum alloy 6061-T6bar stock. The individual blade in each section is slightly offset fromthe adjacent section as shown in FIG. 18A to minimize blade causedpressure perturbation in the discharge region.

[0085] This embodiment as shown in FIG. 18 utilizes two 3 phasebrushless DC motors each with a magnetic rotor contained within ametallic pressure cup which separates the stator portion of the motorsfrom the laser gas environment as described in U.S. Pat. No. 4,950,840.In this embodiment, the pressure cup is thin-walled nickel alloy 400,0.016 inch thick which functions as the laser gas barrier. The twomotors 530 and 532 drive the same shaft and are programmed to rotate inopposite directions. Both motors are sensorless motors (i.e., theyoperate without position sensors). Right motor controller 534 whichcontrols right motor 530 functions as a master controller controllingslave motor controller 536 via analog and digital signals to institutestart/stop, current command, current feedback, etc. Communication withthe laser controller 24A is via a RS-232 serial port into mastercontroller 534.

WATER COOLED PULSE POWER SYSTEM Four Thousand Hz Pulse Power System

[0086] Operation of laser systems in accordance with the presentinvention requires precisely controlled electrical potentials in therange of about 12,000 V to 30,000 V be applied between the electrodes at4,000 Hz (i.e., at intervals of about 250 micro seconds). As indicatedin the background section, in prior art pulse power systems a chargingcapacitor bank is charged to a precisely predetermined control voltageand the discharge is produced by closing a solid state switch whichallows the energy stored on the charging capacitor to ring through acompression-amplification circuit to produce the desired potentialacross the electrodes. The time between the closing of the switch to thecompletion of the discharge is only a few microseconds, (i.e., about 5microseconds) but the charging of C₀ in prior art systems required atime interval much longer than 250 microseconds. It is possible toreduce the charging time by using a larger power supply or several powersupplies in parallel. For example, Applicants have been able to operateat 4,000 Hz using three prior art power supplies arranged in parallel.

[0087] In this preferred embodiment, as shown in FIG. 5 Applicantsutilize the same basic design as in the prior art shown in FIG. 3 forthe portion of the pulse power system downstream of the solid stateswitch, but Applicants utilize a radically different technique forcharging C₀.

Resonant Charging

[0088] Applicants have utilized two types of resonant charging systemsfor very fast charging of C₀. These systems can be described byreference to FIGS. 6A and 6B.

First Resonant Charger

[0089] An electrical circuit showing this preferred resonant charges isshown in FIG. 6A. In this case, a standard dc power supply 200 having a208 VAC/90 amp input and an 800 VDC 50 amp output is used. The powersupply is a dc power supply adjustable from approximately 600 volts to800 volts. The power supply is attached directly to C-1 eliminating theneed for voltage feedback to the supply. When the supply is enabled itturns on and regulates a constant voltage on C-1 capacitor. Theperformance of the system is somewhat independent of the voltageregulation on C-1 therefore only the most basic control loop isnecessary in the power supply. Secondly the supply will be adding energyinto the system whenever the voltage on C-1 falls below the voltagesetting. This allows the power supply the entire time between initiationof laser pulses, (and even during laser pulses), to replenish energytransferred from C-1 to C₀. This further reduces the power supply peakcurrent requirements over the prior art pulse power systems. Thecombination of requiring a supply with the most basic control loop, andminimizing the peak current rating of the supply to the average powerrequirements of the system reduces the power supply cost an estimated50%. Additionally this preferred design provides vendor flexibilitysince constant current, fixed output voltage power supplies are readilyavailable from multiple sources. Such power supplies are available fromsuppliers such as Elgar, Universal Voltronics, Kaiser and EMI.

Control Board

[0090] This power supply continuously charges a 1033 μF capacitor 202 tothe voltage level commanded by the control board 204. The control board204 also commands IGBT switch 206 closed and open to transfer energyfrom capacitor 202 to capacitor 42. Inductor 208 sets up the transfertime constant in conjunction with capacitor 202 and 42 and limits thepeak charging current. Control board 204 receives a voltage feedback 212that is proportional to the voltage on capacitor 42 and a currentfeedback 214 that is proportional to the current flowing throughinductor 208. From these two feedback signals control board 204 cancalculate in real time the final voltage on capacitor 42 should IGBTswitch 206 open at that instant of time. Therefore with a commandvoltage 210 fed into control board 204 a precise calculation can be madeof the stored energy within capacitor 42 and inductor 208 to compare tothe required charge voltage commanded 210. From this calculation, thecontrol board 204 will determine the exact time in the charge cycle toopen IGBT switch 206.

System Accuracy

[0091] After IGBT switch 206 opens the energy stored in the magneticfield of inductor 208 will transfer to capacitor 42 through thefree-wheeling diode path 215. The accuracy of the real time energycalculation will determine the amount of fluctuation dither that willexist on the final voltage on capacitor 42. Due to the extreme chargerate of this system, too much dither may exist to meet a desired systemsregulation need of ±0.05%. If so, additional circuitry may be utilized,such as for example, a de-qing circuit or a bleed-down circuit asdiscussed below.

Second Resonant Charger

[0092] A second resonant charger system is shown in FIG. 6B. Thiscircuit is similar to the one shown in FIG. 6A. The principal circuitelements are:

[0093] I1—A three-phase power supply 300 with a constant DC currentoutput.

[0094] C-1—A source capacitor 302 that is an order of magnitude or morelarger than the existing C0 capacitor 42.

[0095] Q1, Q2, and Q3—Switches to control current flow for charging andmaintaining a regulated voltage on C₀.

[0096] D1, D2, and D3—Provides current single direction flow.

[0097] R1, and R2—Provides voltage feedback to the control circuitry.

[0098] R3—Allows for rapid discharge of the voltage on C₀ in the eventof a small over charge.

[0099] L1—Resonant inductor between C-1 capacitor 302 and C₀ capacitor4-2 to limit current flow and setup charge transfer timing.

[0100] Control Board 304—Commands Q1, Q2, and Q3 open and closed basedupon circuit feedback parameters.

[0101] An example of operation is as follows:

[0102] The difference in the circuit of FIG. 6B from that of 6A is theaddition of switch Q2 and diode D3, known as a De-Qing switch. Thisswitch improves the regulation of the circuit by allowing the controlunit to short out the inductor during the resonant charging process.This “de-qing” prevents additional energy stored in the current of thecharging inductor, L1, from being transferred to capacitor C_(o.)

[0103] Prior to the need for a laser pulse the voltage on C-1 is chargedto 600-800 volts and switches Q1-Q3 are open. Upon command from thelaser, Q1 would close. At this time current would flow from C-1 to C₀through the charge inductor L1. As described in the previous section, acalculator on the control board would evaluate the voltage on C₀ and thecurrent flowing in L1 relative to a command voltage set point from thelaser. Q1 will open when the voltage on C₀ plus the equivalent energystored in inductor L1 equals the desired command voltage. Thecalculation is:

V _(f) =[V _(C0s) ²+((L ₁ *I _(LIs) ²)/C ₀)]^(0.5)

[0104] Where:

[0105] V_(f)=The voltage on C₀ after Q1 opens and the current in L1 goesto zero.

[0106] V_(C0s)=The voltage on C₀ when Q1 opens.

[0107] I_(L1s)=The current flowing through L₁ when Q1 opens.

[0108] After Q1 opens the energy stored in L1 starts transferring to C₀through D2 until the voltage on C₀ approximately equals the commandvoltage. At this time Q2 closes and current stops flowing to C₀ and isdirected through D3. In addition to the “de-qing” circuit, Q3 and R3from a bleed-down circuit to allow additional fine regulation of thevoltage on C_(o.)

[0109] Switch Q3 of bleed down circuit 216 will be commanded closed bythe control board when current flowing through inductor L1 stops and thevoltage on C0 will be bled down to the desired control voltage; thenswitch Q3 is opened. The time constant of capacitor C_(o) and resistorR3 should be sufficiently fast to bleed down capacitor C_(o) to thecommand voltage without being an appreciable amount of the total chargecycle.

[0110] As a result, the resonant charger can be configured with threelevels of regulation control. Somewhat crude regulation is provided bythe energy calculator and the opening of switch Q1 during the chargingcycle. As the voltage on C₀ nears the target value, the de-qing switchis closed, stopping the resonant charging when the voltage on C_(o) isat or slightly above the target value. In a preferred embodiment, theswitch Q1 and the de-qing switch is used to provide regulation withaccuracy better than +/−0.1%. If additional regulation is required, thethird control over the voltage regulation could be utilized. This is thebleed-down circuit of switch Q3 and R3 (shown at 216 in FIG. 6B) todischarge C₀ down to the precise target value.

Improvements Downstream of C₀

[0111] As indicated above, the pulse power system of the presentinvention utilizes the same basic design as was used in the prior artsystems described in FIG. 3. However, some significant improvements inthat basic design were required for the approximate factor of 3 increasein heat load resulting from the greatly increased repetition rate. Theseimprovements are discussed below.

Detailed Commutator and Compression Head Description

[0112] The principal components of commutator 40 and compression head 60are shown in FIG. 3 and are discussed in the Background section withregard to the operation of the system. In this section, we describedetails of fabrication of the commutator and the compression head.

Solid State Switch

[0113] Solid state switch 46 is an P/N CM 800 HA-34H IGBT switchprovided by Powerex, Inc. with offices in Youngwood, Pa. In a preferredembodiment, two such switches are used in parallel.

Inductors

[0114] Inductors 48, 54 and 64 are saturable inductors similiar to thoseused in prior systems as described in U.S. Pat. Nos. 5,448,580 and5,315,611 which are incorporated herein by reference. FIG. 7 shows apreferred design of the L_(o) inductor 48. In this inductor fourconductors from the two IGBT switches 46B pass through sixteen ferritetoroids 49 to form part 48A an 8 inch long hollow cylinder of very highpermability material with an ID of about 1 inch and an Od of about 1.5inch. Each of the four conductors are then wrapped twice around aninsulating doughnut shaped core to form part 48B. The four conductorsthen connect to a plate which is in turn connected to the high voltageside of the C₁ capacitor bank 52.

[0115] A preferred sketch of saturable inductor 54 is shown in FIG. 8.In this case, the inductor is a single turn geometry where the assemblytop and bottom lids 541 and 542 and center mandrel 543, all at highvoltage, form the single turn through the inductor magnetic cores. Theouter housing 545 is at ground potential. The magnetic cores are 0.0005″thick tape wound 50-50% Ni-Fe alloy provided by Magnetics of Butler, Pa.or National Arnold of Adelanto, Calif. Fins 546 on the inductor housingfacilitate transfer of internally dissipated heat to forced air cooling.In addition, a ceramic disk (not shown) is mounted underneath thereactor bottom lid to help transfer heat from the center section of theassembly to the module chassis base plate. FIG. 8 also shows the highvoltage connections to one of the capacitors of the C₁ capacitor bank 52and to a high voltage lead on one of the induction units of the 1:25step up pulse transformer 56. The housing 545 is connected to the groundlead of unit 56.

[0116] A top and section view of the saturable inductor 64 is shownrespectively in FIGS. 9A and 9B. In the inductors of this embodiment,flux excluding metal pieces 301, 302, 303 and 304 are added as shown inFIG. 9B in order to reduce the leakage flux in the inductors. These fluxexcluding pieces substantially reduce the area which the magnetic fluxcan penetrate and therefore help to minimize the saturated inductance ofthe inductor. The current makes five loops through vertical conductorrods in the inductor assembly around magnetic core 307. The currententers at 305 travels down a large diameter conductor in the centerlabeled “1” and up six smaller conductors on the circumference alsolabeled “1” as shown in FIG. 9A. The current then flows down twoconductors labeled 2 on the inside, then up the six conductors labeled 2on the outside then down flux exclusion metal on the inside then up thesix conductors labeled 3 on the outside, then down the two conductorslabeled 3 on the inside, then up the six conductors labeled 4 on theoutside, then down the conductor labeled 4 on the inside. The fluxexclusion metal components are held at half the full pulsed voltageacross the conductor allowing a reduction in the safe hold-off spacingbetween the flux exclusion metal parts and the metal rods of the otherturns. The magnetic core 307 is made up of three coils 307A, B and Cformed by windings of 0.0005″ thick tape 80-20% Ni—Fe alloy provided byMagnetics, Inc. of Butler, Pa. or National Arnold of Adelanto, Calif.The reader should note that nano-crystoline materials such as VITROPEAN™available from VACUUM SCHITELZE GmbH, Germany and FINEMET™ from HitachiMekels, Japan could be used for inductors 54 and 64.

[0117] In prior art pulse power systems, oil leakage from electricalcomponents has been a potential problem. In this preferred embodiment,oil insulated components are limited to the saturable inductors.Furthermore, the saturable inductor 64 as shown in FIG. 9B is housed ina pot type oil containing housing in which all seal connections arelocated above the oil level to substantially eliminate the possibilityof oil leakage. For example, the lowest seal in inductor 64 is shown at308 in FIG. 9B. Since the normal oil level is below the top lip of thehousing 306, it is almost impossible for oil to leak outside theassembly as long as the housing is maintained in an upright condition.

Capacitors

[0118] Capacitor banks 42, 52, 62 and 82 (i.e., C₀, C₁, C_(p-1) andC_(p)) as shown in FIG. 5 are all comprised of banks of off-the-shelfcapacitors connected in parallel. Capacitors 42 and 52 are film typecapacitors available from suppliers such as Vishay Roederstein withoffices in Statesville, N.C. or Wima of Germany. Applicants preferredmethod of connecting the capacitors and inductors is to solder them topositive and negative terminals on special printed circuit board havingheavy nickel coated copper leads in a manner similar to that describedin U.S. Pat. No. 5,448,580. Capacitor bank 62 and 64 is typicallycomposed of a parallel array of high voltage ceramic capacitors fromvendors such as Murata or TDK, both of Japan. In a preferred embodimentfor use on this ArF laser, capacitor bank 82 (i.e., C_(p)) comprised ofa bank of thirty three 0.3 nF capacitors for a capacitance of 9.9 nF;C_(p-1) is comprised of a bank of twenty four 0.40 nF capacitors for atotal capacitance of 9.6 nF; C₁ is a 5.7 μF capacitor bank and C₀ is a5.3 μF capacitor bank.

Pulse Transformer

[0119] Pulse transformer 56 is also similar to the pulse transformerdescribed in U.S. Pat. Nos. 5,448,580 and 5,313,481; however, the pulsetransformers of the present embodiment has only a single turn in thesecondary winding and 24 induction units equivalent to {fraction (1/24)} of a single primary turn for an equivalent step-up ratio of 1:24. Adrawing of pulse transformer 56 is shown in FIG. 10. Each of the 24induction units comprise an aluminum spool 56A having two flanges (eachwith a flat edge with threaded bolt holes) which are bolted to positiveand negative terminals on printed circuit board 56B as shown along thebottom edge of FIG. 10. (The negative terminals are the high voltageterminals of the twenty four primary windings.)

[0120] Insulators 56C separates the positive terminal of each spool fromthe negative terminal of the adjacent spool. Between the flanges of thespool is a hollow cylinder 1{fraction (1/16)} inches long with a 0.875OD with a wall thickness of about {fraction (1/32)} inch. The spool iswrapped with one inch wide, 0.7 mil thick Metglas™ 2605 S3A and a 0.1mil thick mylar film until the OD of the insulated Metglas™ wrapping is2.24 inches. A prospective view of a single wrapped spool forming oneprimary winding is shown in FIG. 10A.

[0121] The secondary of the transformer is a single OD stainless steelrod mounted within a tight fitting insulating tube of PTFE (Teflon®).The winding is in four sections as shown in FIG. 10. The low voltage endof stainless steel secondary shown as 56D in FIG. 10 is tied to theprimary HV lead on printed circuit board 56B at 56E, the high voltageterminal is shown at 56F. As a result, the transformer assumes anauto-transformer configuration and the step-up ratio becomes 1:25instead of 1:24. Thus, an approximately −1400 volt pulse between the +and − terminals of the induction units will produce an approximately−35,000 volt pulse at terminal 56F on the secondary side. This singleturn secondary winding design provides very low leakage inductancepermitting extremely fast output rise time.

Details of Laser Chamber Electrical Components

[0122] The Cp capacitor 82 is comprised of a bank of thirty-three 0.3 nfcapacitors mounted on top of the chamber pressure vessel. (Typically anArF laser is operated with a lasing gas made up of 3.5% argon, 0.1%fluorine, and the remainder neon.) The electrodes are about 28 incheslong which are separated by about 0.5 to 1.0 inch preferably about ⅝inch. Preferred electrodes are described below. In this embodiment, thetop electrode is referred to as the cathode and the bottom electrode isconnected to ground as indicated in FIG. 5 and is referred to as theanode.

Water Cooling of Components

[0123] To accommodate greater heat loads a water cooling presented herewhich is better able to support operation at this higher average powermode in addition to the normal forced air cooling provided by coolingfans inside the laser cabinet.

[0124] One disadvantage of water cooling has traditionally been thepossibility of leaks near the electrical components or high voltagewiring. This specific embodiment substantially avoids that potentialissue by utilizing a single solid piece of cooling tubing that is routedwithin a module to cool those components that normally dissipate themajority of the heat deposited in the module. Since no joints orconnections exist inside the module enclosure and the cooling tubing isa continuous piece of solid metal (e.g. copper, stainless steel, etc.),the chances of a leak occurring within the module are greatlydiminished. Module connections to the cooling water are therefore madeoutside the assembly sheet metal enclosure where the cooling tubingmates with a quick-disconnect type connector.

Detailed Commutator Description

[0125] In the case of the commutator module a water cooled saturableinductor 54A is provided as shown in FIG. 11A which is similar to theinductor 54 shown in FIG. 8 except the fins of 54 are replaced with awater cooled jacket 54A1 as shown in FIG. 11A. The cooling line 54A2 isrouted within the module to wrap around jacket 54A1 and through aluminumbase plate where the IGBT switches and Series diodes are mounted. Thesethree components make up the majority of the power dissipation withinthe module. Other items that also dissipate heat (snubber diodes andresistors, capacitors, etc.) are cooled by forced air provided by thetwo fans in the rear of the module.

[0126] Since the jacket 54A1 is held at ground potential, there are novoltage isolation issues in directly attaching the cooling tubing to thereactor housing. This is done by press-fitting the tubing into adovetail groove cut in the outside of the housing as shown at 54A3 andusing a thermally conductive compound to aid in making good thermalcontact between the cooling tubing and the housing.

Cooling High Voltage Components

[0127] Although the IGBT switches “float” at high voltage, they aremounted on an aluminum base electrically isolated from the switches by a{fraction (1/16)} inch thick alumina plate. The aluminum base platewhich functions as a heat sink and operates at ground potential and ismuch easier to cool since high voltage isolation is not required in thecooling circuit. A drawing of a water cooled aluminum base plate isshown in FIG. 7A. In this case, the cooling tubing is pressed into agroove in an aluminum base on which the IGBT's are mounted. As with theinductor 54 a, thermally conductive compound is used to improve theoverall joint between the tubing and the base plate.

[0128] The series diodes also “float” at high potential during normaloperation. In this case, the diode housing typically used in the designprovides no high voltage isolation. To provide this necessaryinsulation, the diode “hockey puck” package is clamped within a heatsink assembly which is then mounted on top of a ceramic base that isthen mounted on top of the water-cooled aluminum base plate. The ceramicbase is just thick enough to provide the necessary electrical isolationbut not too thick to incur more than necessary thermal impedance. Forthis specific design, the ceramic is {fraction (1/16)}″ thick aluminaalthough other more exotic materials, such as beryllia, can also be usedto further reduce the thermal impedance between the diode junction andthe cooling water.

[0129] A second embodiment of a water cooled commutator utilizes assingle cold plate assembly which is attached to the chassis baseplatefor the IGBT's and the diodes. The cold plate may be fabricated bybrazing single piece nickel tubing to two aluminum “top” and “bottom”plates. As described above, the IGBT's and diodes are designed totransfer their heat into the cold plate by use of the previouslymentioned ceramic disks underneath the assembly. In a preferredembodiment of this invention, the cold plate cooling method is also usedto cool the IGBT and the diodes in the resonant charger. Thermallyconductive rods or a heat pipe can also be used to transfer heat fromthe outside housing to the chassis plate.

Detailed Compression Head Description

[0130] The water-cooled compression head is similar in the electricaldesign to a prior art air-cooled version (the same type ceramiccapacitors are used and similar material is used in the reactordesigns). The primary differences in this case are that the module mustrun at higher rep-rates and therefore, higher average power. In the caseof the compression head module, the majority of the heat is dissipatedwithin the modified saturable inductor 64A. Cooling the subassembly isnot a simple matter since the entire housing operates with short pulsesof very high voltages. The solution to this issue as shown in FIGS. 12,12A and 12B is to inductively isolate the housing from ground potential.This inductance is provided by wrapping the cooling tubing around twocylindrical forms that contain a ferrite magnetic core. Both the inputand output cooling lines are coiled around cylindrical portions of aferrite core formed of the two cylindrical portions and the two ferriteblocks as shown in FIGS. 12, 12A and 12B.

[0131] The ferrite pieces are made from CN-20 material manufactured byCeramic Magnetics, Inc. of Fairfield, N.J. A single piece of coppertubing (0.187″ diameter) is press fit and wound onto one winding form,around the housing 64A1 of inductor 64A and around the second windingform. Sufficient length is left at the ends to extend through fittingsin the compression head sheet metal cover such that no cooling tubingjoints exist within the chassis.

[0132] The inductor 64A comprises a dovetail groove as shown at 64A2similar to that used in the water-cooled commutator first stage reactorhousing. This housing is much the same as previous air-cooled versionswith the exception of the dovetail groove. The copper cooling-watertubing is press fit into this groove in order to make a good thermalconnection between the housing and the cooling-water tubing. Thermallyconductive compound is also added to minimize the thermal impedance.

[0133] The electrical design of inductor 64A is changed slightly fromthat of 64 shown in FIGS. 9A and 9B. Inductor 64A provides only twoloops (instead of five loops) around magnetic core 64A3 which iscomprised of four coils of tape (instead of three).

[0134] As a result of this water-cooled tubing conductive path from theoutput potential to ground, the bias current circuit is now slightlydifferent. As before, bias current is supplied by a dc-dc converter inthe commutator through a cable into the compression head. The currentpasses through the “positive” bias inductor L_(B2) and is connected tothe Cp-1 voltage node. The current then splits with a portion returningto the commutator through the HV cable (passing through the transformersecondary to ground and back to the dc-dc converter). The other portionpasses through the compression head reactor Lp-1 (to bias the magneticswitch) and then through the cooling-water tubing “negative” biasinductor L_(B3) and back to ground and the dc-dc converter. By balancingthe resistance in each leg, the designer is able to ensure thatsufficient bias current is available for both the compression headreactor and the commutator transformer. The “positive” bias inductorL_(B2) is made very similarly to the “negative” bias inductor L_(B3). Inthis case, the same ferrite bars and blocks are used as a magnetic core.However, two 0.125″ thick plastic spacers are used to create an air gapin the magnetic circuit so that the cores do not saturate with the dccurrent. Instead of winding the inductor with cooling-water tubing, 18AWG teflon wire is wound around the forms.

Quick Connections

[0135] In this preferred embodiment, three of the pulse power electricalmodules utilize blind mate electrical connections so that all electricalconnections to the portions of the laser system are made merely bysliding the module into its place in the laser cabinet. These are the ACdistribution module, the power supply module and the resonant chargermodule. In each case a male or female plug on the module mates with theopposite sex plug mounted at the back of the cabinet. In each case twoapproximately 3-inch end tapered pins on the module guide the moduleinto its precise position so that the electrical plugs properly mate.The blind mate connectors such as AMP Model No. 194242-1 arecommercially available from AMP, Inc. with offices in Harrisburg, Pa. Inthis embodiment connectors are for 208 volt AC, 400 volt AC, 800 volt DC(power supply out and resonant charger in) and several signal voltages.These blind mate connections permit these modules to be removed forservicing and replacing in a few seconds or minutes. In this embodimentblind mate connections are not used for the commutator module since theoutput voltage of the module is in the range of 20,000 to 30,000 volts.Instead, a typical high voltage connector is used.

Discharge Components

[0136]FIGS. 13 and 13A show details of an improved dischargeconfiguration utilized in preferred embodiments of the presentinvention. This configuration includes an electrode configuration thatApplicants call a blade-dielectric electrode. In this design, the anode540 comprises a blunt blade shaped electrode 542 with dielectric spaces544 mounted on both sides of the anode as shown to improve the gas flowin the discharge region. The anode is 26.4 inches long and 0.439 incheshigh. It is 0.284 inches wide at the bottom and 0.141 inches wide at thetop. It is attached to flow shaping anode support bar 546 with screwsthrough sockets that allow differential thermal expansion of theelectrode from its center position. The anode is comprised of a copperbased alloy preferably C36000, C95400, or C19400. Cathode 541 has across section shape as shown in FIG. 25A which is slightly pointed atthe anode facing position. A preferred cathode material is C36000.Additional details of this blade dielectric configuration are providedin U.S. patent application Ser. No. 09/768,753 incorporated herein byreference. The current return 548 in this configuration is comprised ofa single long section of thin (about {fraction (1/16)}″ diameter) copperor brass wire formed into a whale bone shaped with 27 ribs equallyspaced along the length of electrode 542, the cross section of which isshown in FIGS. 13 and 13A. The wire is clamped into line grooves at thebottom of anode and semi-circular grooves at the chamber top insidesurface.

ULTRA FAST WAVEMETER WITH FAST CONTROL ALGORITHM Controlling PulseEnergy, Wavelength and Bandwidth

[0137] Prior art excimer lasers used for integrated circuit lithographyare subject to tight specifications on laser beam parameters. This hastypically required the measurement of pulse energy, bandwidth and centerwavelength for every pulse and feedback control of pulse energy andbandwidth. In prior art devices the feedback control of pulse energy hasbeen on a pulse-to-pulse basis, i.e., the pulse energy of each pulse ismeasured quickly enough so that the resulting data can be used in thecontrol algorithm to control the energy of the immediately followingpulse. For a 1,000 Hz system this means the measurement and the controlfor the next pulse must take less than {fraction (1/1000)} second. For a4000 Hz system speeds need to be four times as fast. A technique forcontrolling center wavelength and measuring wavelength and bandwidth isdescribed in U.S. Pat. No. 5,025,455 System, and Method of Regulatingthe Wavelength of a Light Beam and in U.S. Pat. No. 5,978,394,Wavelength and System for an Excimer Laser. These patents areincorporated herein by reference.

[0138] Wavelength and bandwidths have been measured on a pulse to pulsebasis for every pulse, but typically the feedback control of wavelengthhas taken about 7 milli-seconds because prior art techniques forcontrolling center wavelength have taken several milli-seconds. Fastercontrol is needed.

Preferred Embodiment for Fast Measurement and Control of Beam Parameters

[0139] A preferred embodiment of the present invention is an excimerlaser system capable of operation in the range of 4,000 Hz to 6,000 Hzwith very fast measurement of laser beam parameters and very fastcontrol of pulse energy and center wavelength. The beam parametermeasurement and control for this laser is described below.

[0140] The wavemeter used in the present embodiment is similar to theone described in U.S. Pat. No. 5,978,394 and some of the descriptionbelow is extracted from that patent.

Measuring Beam Parameters

[0141]FIG. 14 shows the layouts of a preferred wavemeter unit 120, anabsolute wavelength reference calibration unit 190, and a wavemeterprocessor 197.

[0142] The optical equipment in these units measure pulse energy,wavelength and bandwidth. These measurements are used with feedbackcircuits to maintain pulse energy and wavelength within desired limits.The equipment calibrates itself by reference to an atomic referencesource on the command from the laser system control processor.

[0143] As shown in FIG. 14, the laser output beam intersects partiallyreflecting mirror 170, which passes about 95.5% of the beam energy asoutput beam 33 and reflects about 4.5% for pulse energy, wavelength andbandwidth measurement.

Pulse Energy

[0144] About 4% of the reflected beam is reflected by mirror 171 toenergy detector 172 which comprises a very fast photo diode 69 which isable to measure the energy of individual pulses occurring at the rate of4,000 pulses per second. The pulse energy is about 5 mJ, and the outputof detector 69 is fed to a computer controller which uses a specialalgorithm to adjust the laser charging voltage to precisely control thepulse energy of future pulses based on stored pulse energy data in orderto limit the variation of the energy of individual pulses and theintegrated energy of bursts of pulses.

Linear Photo Diode Array

[0145] The photo sensitive surface of linear photo diode array 180 isdepicted in detail in FIG. 14A. The array is an integrated circuit chipcomprising 1024 separate photo diode integrated circuits and anassociated sample and hold readout circuit. The photo diodes are on a 25micrometer pitch for a total length of 25.6 mm (about one inch). Eachphoto diode is 500 micrometer long.

[0146] Photo diode arrays such as this are available from severalsources. A preferred supplier is Hamamatsu. In our preferred embodiment,we use a Model S3903-1024Q which can be read at the rate of up to 4×10⁶pixels/sec on a FIFO basis in which complete 1024 pixel scans can beread at rates of 4,000 Hz or greater. The PDA is designed for 2×10⁶pixel/sec operation but Applicants have found that it can beover-clocked to run much faster, i.e., up to 4×10⁶ pixel/sec. For pulserates greater than 4,000 Hz, Applicants can use the same PDA but only afraction (such as 60%) of the pixels are normally read on each scan.

Coarse Wavelength Measurement

[0147] About 4% of the beam which passes through mirror 171 is reflectedby mirror 173 through slit 177 to mirror 174, to mirror 175, back tomirror 174 and onto echelle grating 176. The beam is collimated by lens178 having a focal length of 458.4 mm. Light reflected from grating 176passes back through lens 178, is reflected again from mirrors 174, 175and 174 again, and then is reflected from mirror 179 and focused ontothe left side of 1024-pixel linear photo diode array 180 in the regionof pixel 600 to pixel 950 as shown in the upper part of FIG. 14B (Pixels0-599 are reserved for fine wavelength measurement and bandwidth.) Thespatial position of the beam on the photo diode array is a coarsemeasure of the relative nominal wavelength of the output beam. Forexample, as shown in FIG. 14B, light in the wavelength range of about193.350 pm would be focused on pixel 750 and its neighbors.

Calculation of Coarse Wavelength

[0148] The coarse wavelength optics in wavemeter module 120 produces arectangular image of about 0.25 mm×3 mm on the left side of photo diodearray 180. The ten or eleven illuminated photo diodes will generatesignals in proportion to the intensity of the illumination received (asindicated in FIG. 14C) and the signals are read and digitized by aprocessor in wavemeter controller 197. Using this information and aninterpolation algorithm controller 197 calculates the center position ofthe image.

[0149] This position (measured in pixels) is converted into a coarsewavelength value using two calibration coefficients and assuming alinear relationship between position and wavelength. These calibrationcoefficients are determined by reference to an atomic wavelengthreference source as described below. For example, the relationshipbetween image position and wavelength might be the following algorithm:

λ=(2.3 pm/pixel)P+191,625 pm

[0150] where P=coarse image central positions.

[0151] Alternatively, additional precision could be added if desired byadding a second order term such as “+( ) P^(2.)

Fine Wavelength Measurement

[0152] About 95% of the beam which passes through mirror 173 as shown inFIG. 16 is reflected off mirror 182 through lens 183 onto a diffuser(preferably a diffraction diffuser as explained in a following sectionentitled “Improved Etalon”) at the input to etalon assembly 184. Thebeam exiting etalon 184 is focused by a 458.4 mm focal length lens inthe etalon assembly and produces interference fringes on the middle andright side of linear photo diode array 180 after being reflected off twomirrors as shown in FIG. 14.

[0153] The spectrometer must measure wavelength and bandwidthsubstantially in real time. Because the laser repetition rate may be4,000 Hz to 6,000 Hz, it is necessary to use algorithms which areaccurate but not computationally intensive in order to achieve thedesired performance with economical and compact processing electronics.Calculational algorithm therefore preferably should use integer asopposed to floating point math, and mathematical operations shouldpreferably be computation efficient (no use of square root, sine, log,etc.).

[0154] The specific details of a preferred algorithm used in thispreferred embodiment will now be described. FIG. 14D is a curve with 5peaks as shown which represents a typical etalon fringe signal asmeasured by linear photo diode array 180. The central peak is drawnlower in height than the others. As different wavelengths of light enterthe etalon, the central peak will rise and fall, sometimes going tozero. This aspect renders the central peak unsuitable for the wavelengthmeasurements. The other peaks will move toward or away from the centralpeak in response to changes in wavelength, so the position of thesepeaks can be used to determine the wavelength, while their widthmeasures the bandwidth of the laser. Two regions, each labeled datawindow, are shown in FIG. 14D. The data windows are located so that thefringe nearest the central peak is normally used for the analysis.However, when the wavelength changes to move the fringe too close to thecentral peak (which will cause distortion and resulting errors), thefirst peak is outside the window, but the second closest peak will beinside the window, and the software causes the processor in controlmodule 197 to use the second peak. Conversely, when the wavelengthshifts to move the current peak outside the data window away from thecentral peak the software will jump to an inner fringe within the datawindow. The data windows are also depicted on FIG. 14B.

[0155] For very fast computation of bandwidth for each pulse atrepetition rates up to the range of 4,000 Hz to 6,000 Hz a preferredembodiment uses the hardware identified in FIG. 15. The hardwareincludes a microprocessor 400, Model MPC 823 supplied by Motorola withoffices in Phoenix, Ariz.; a programmable logic device 402, Model EP6016QC240 supplied by Altera with offices in San Jose, Calif.; anexecutive and data memory bank 404; a special very fast RAM 406 fortemporary storage of photodiode array data in table form; a third 4×1024pixel RAM memory bank 408 operating as a memory buffer; and an analog todigital converter 410.

[0156] As explained in U.S. Pat. Nos. 5,025,446 and 5,978,394, prior artdevices were required to analyze a large mass of PDA data pixelintensity data representing interference fringes produced by etalon 184an photodiode array 180 in order to determine center line wavelength andbandwidth. This was a relatively time consuming process even with acomputer processor because about 400 pixel intensity values had to beanalyzed to look for and describe the etalon fringes for eachcalculation of wavelength and bandwidth. A preferred embodiment of thepresent invention greatly speeds up this process by providing aprocessor for finding the important fringes which operates in parallelwith the processor calculating the wavelength information.

[0157] The basic technique is to use programmable logic device 402 tocontinuously produce a fringe data table from the PDA pixel data as thepixel data are produced. Logic device 402 also identifies which of thesets of fringe data represent fringe data of interest. Then when acalculation of center wavelength and bandwidth are needed,microprocessor merely picks up the data from the identified pixels ofinterest and calculates the needed values of center wavelength andbandwidth. This process reduces the calculation time for microprocessorby about a factor of about 10.

[0158] Specific steps in the process of calculating center wavelengthand bandwidth are as follows:

[0159] 1) With PDA 180 clocked to operate at 2.5 MHz, PDA 180 isdirected by processor 400 to collect data at a from pixels 1 to 600 at ascan rate of 4,000 Hz and to read pixels 1 to 1028 at a rate of 100 Hz.

[0160] 2) The analog pixel intensity data produced by PDA 180 isconverted from analog intensity values into digital 8 bit values (0 to255) by analog to digital converter 410 and the digital data are storedtemporily in RAM buffer 408 as 8 bit values representing intensity ateach pixel of photodiode array 180.

[0161] 3) Programmable logic device 402 analyzes the data passing out ofRAM buffer 408 continuously on an almost real time basis looking forfringes, stores all the data in RAM memory 406, identifies all fringesfor each pulse, produces a table of fringes for each pulse and storesthe tables in RAM 406, and identifies for further analysis one best setof two fringes for each pulse. The technique used by logic device 402 isas follows:

[0162] A) PLD 402 analyzes each pixel value coming through buffer 408 todetermine if it exceeds an intensity threshold while keeping track ofthe minimum pixel intensity value. If the threshold is exceeded this isan indication that a fringe peak is coming. The PLD identifies the firstpixel above threshold as the “rising edge” pixel number and saves theminimum pixel value of the pixels preceeding the “rising edge” pixel.The intensity value of this pixel is identified as the “minimum” of thefringe.

[0163] B) PLD 402 then monitors subsequent pixel intensity values tosearch for the peak of the fringe. It does this by keeping track of thehighest intensity value until the intensity drops below the thresholdintensity.

[0164] C) When a pixel having a value below threshold is found, the PLDidentifies it as the falling edge pixel number and saves the maximumvalue. The PLD then calculates the “width” of the fringe by substractingthe rising edge pixel number from the falling edge pixel number.

[0165] D) The four values of rising edge pixel number, maximum fringeintensity, minimum fringe intensity and width of the fringe are storedin the circular table of fringes section of RAM memory bank 406. Datarepresenting up to 15 fringes can be stored for each pulse although mostpulses only produce 2 to 5 fringes in the two windows.

[0166] E) PLD 402 also is programmed to identify with respect to eachpulse the “best” two fringes for each pulse. It does this by identifyingthe last fringe completely within the 0 to 199 window and the firstfringe completely within the 400 to 599 window.

[0167] The total time required after a pulse for (1) the collection ofthe pixel data, and (2) the formation of the circular table of fringesfor the pulse is only about 200 micro seconds. The principal time savingadvantages of this technique is that the search for fringes is occurringas the fringe data is being read out, digitized and stored. Once the twobest fringes are identified for a particular pulse, microprocessor 400secures the raw pixel data in the region of the two fringes from RAMmemory bank 406 and calculates from that data the bandwidth and centerwavelength. The calculation is as follows:

[0168] Typical shape of the etalon fringes are shown in FIG. 14D. Basedon the prior work of PLD 402 the fringe having a maximum at about pixel180 and the fringe having a maximum at about pixel 450 will beidentified to microprocessor 400. The pixel data surrounding these twomaxima are analyzed by microprocessor 400 to define the shape andlocation of the fringe. This is done as follows:

[0169] A) A half maximum value is determined by subtracting the fringeminimum from the fringe maximum dividing the difference by 2 and addingthe result to the fringe minimum. For each rising edge and each fallingedge of the two fringes the two pixels having values of closest aboveand closest below the half maximum value. Microprocessor thenextrapolates between the two pixel values in each case to define the endpoints of D1 and D2 as shown in FIG. 18B with a precision of {fraction(1/32)} pixel. From these values the inner diameter D1 and the outerdiameter D2 of the circular fringe are determined.

Fine Wavelength Calculation

[0170] The fine wavelength calculation is made using the coursewavelength measured value and the measured values of D1 and D2.

[0171] The basic equation for wavelength is:

λ=(2*n*d/m) cos(R/f)  (1)

[0172] where

[0173] λ is the wavelength, in picometers,

[0174] n is the internal index of refraction of the etalon, about1.0003,

[0175] d is the etalon spacing, about 1542 um for KrF lasers and about934 μm for ArF lasers, controlled to +/−1 um,

[0176] m is the order, the integral number of wavelengths at the fringepeak, about 12440,

[0177] R is the fringe radius, 130 to 280 PDA pixels, a pixel being 25microns,

[0178] f is the focal distance from the lens to the PDA plane.

[0179] Expanding the cos term and discarding high order terms that arenegligibly small yields:

λ=(2*n*d/m)[1−(½)(R/f)²]  (2)

[0180] Restating the equation in terms of diameter D=2*R yields:

λ=(2*n*d/m)[1−(⅛)(D/f)²]  (3)

[0181] The wavemeter's principal task is to calculate λ from D. Thisrequires knowing f, n, d and m. Since n and d are both intrinsic to theetalon we combine them into a single calibration constant named ND. Weconsider f to be another calibration constant named FD with units ofpixels to match the units of D for a pure ratio. The integer order mvaries depending on the wavelength and which fringe pair we choose. m isdetermined using the coarse fringe wavelength, which is sufficientlyaccurate for the purpose.

[0182] A couple of nice things about these equations is that all the bignumbers are positive values. The WCM's microcontroller is capable ofcalculating this while maintaining nearly 32 bits of precision. We referto the bracketed terms as FRAC.

FRAC=[1−(⅛)(D/FD)²]  (4)

[0183] Internally FRAC is represented as an unsigned 32 bit value withits radix point to the left of the most significant bit. FRAC is alwaysjust slightly less than one, so we get maximal precision there. FRACranges from [1-120E-6] to [1-25E-6] for D range of {560 ˜260} pixels.

[0184] When the ND calibration is entered, the wavemeter calculates aninternal unsigned 64 bit value named 2ND=2*ND with internal wavelengthunits of femtometers (fm)=10^ −15 meter=0.001 pm. Internally werepresent the wavelength λ as FWL for the fine wavelength, also in fmunits. Restating the equation in terms of these variables:

FWL=FRAC*2ND/m  (5)

[0185] The arithmetic handles the radix point shift in FRAC yielding FWLin fm. We solve for m by shuffling the equation and plugging in theknown coarse wavelength named CWL, also in fm units:

m=nearest integer (FRAC*2ND/CWL)  (6)

[0186] Taking the nearest integer is equivalent to adding or subtractingFSRs in the old scheme until the nearest fine wavelength to the coarsewavelength was reached. Calculate wavelength by solving equation (4)then equation (6) then equation (5). We calculate WL separately for theinner and outer diameters. The average is the line center wavelength,the difference is the linewidth.

Bandwidth Calculation

[0187] The bandwidth of the laser is computed as (λ₂−λ₁)/2. A fixedcorrection factor is applied to account for the intrinsic width of theetalon peak adding to the true laser bandwidth. Mathematically, adeconvolution algorithm is the formalism for removing the etalonintrinsic width from the measured width, but this would be far toocomputation-intensive, so a fixed correction Δλ∈ is subtracted, whichprovides sufficient accuracy. Therefore, the bandwidth is:${\Delta\lambda} = {\left( \frac{D_{2} - D_{1}}{2} \right) - {\Delta\lambda\varepsilon}}$

[0188] Δλ∈ depends on both the etalon specifications and the true laserbandwidth. It typically lies in the range of 0.1-1 pm for theapplication described here.

Improved Etalon

[0189] This embodiment utilizes an improved etalon. Conventional etalonmounting schemes typically employ an elastomer to mount the opticalelements to the surrounding structure, to constrain the position of theelements but minimize forces applied to the elements. A compoundcommonly used for this is room-temperature vulcanizing silicone (RTV).However, various organic vapors emitted from these elastomers candeposit onto the optical surfaces, degrading their performance. In orderto prolong etalon performance lifetime, it is desirable to mount theetalon in a sealed enclosure that does not contain any elastomercompounds.

[0190] A preferred embodiment includes an improved etalon assembly shownat 184 in FIGS. 14 and 14E. The fused silica etalon 79 shown in FIG. 14Gitself is comprised of a top plate 80 having a flange 81 and a lowerplate 82, both plates being comprised of premium grade fused silica. Theetalon is designed to produce fringes having free spectral range of20.00 pm at 193.35 nm when surrounded by gas with an index of refractionof 1.0003 and a finesse equal to or greater than 25. Three fused silicaspacers 83 with ultra low thermal expansion separate the plates and are934 micrometer ±1 micrometer thick. These hold the etalon together byoptical contact using a technique well known in the optics manufacturingart. The reflectance of the inside surfaces of the etalon are each about88 percent and the outside surfaces are anti-reflection coated. Thetransmission of the etalon is about 50 percent.

[0191] The etalon 79 is held in place in aluminum housing 84 only bygravity and three low force springs 86 pressing the flange against threepads not shown but positioned on 120 degree centers under the bottomedge of flange 81 at the radial location indicated by leader 85. Aclearance of only 0.004 inch along the top edge of flange 81 at 87assures that the etalon will remain approximately in its properposition. This close tolerance fit also ensures that if any shock orimpulse is transferred to the etalon system through the mounting, therelative velocities between the optical components and the housingcontact points will be kept to a minimum. Other optical components ofetalon assembly 184 include diffuser 88, window 89 and focusing lens 90having a focal length of 458.4 mm.

[0192] The diffuser 88 may be a standard prior art diffuser commonlyused up-stream of an etalon to produce a great variety of incidentangles needed for the proper operation of the etalon. A problem withprior art diffusers is that about 90 percent of the light passingthrough the diffuser is not at a useful angle and consequently is notfocused on the photo diode array. This wasted light, however, adds tothe heating of the optical system and can contribute to degradation ofoptical surfaces. In a much preferred embodiment, a diffractive lensarray is used as the diffuser 88. With this type of diffuser, a patternis produced in the diffractive lens array which scatters the lightthoroughly but only within an angle of about 5 degrees. The result isthat about 90 percent of the light falling on the etalon is incident atuseful angles and a much greater portion of the light incident on theetalon is ultimately detected by the photo diode array. The result isthe light incident on the etalon can be greatly reduced which greatlyincreases optical component life. Applicants estimate that the incidentlight can be reduced to less than 5% or 10% of prior art values withequivalent light on the photo diode array.

Better Collimation with Diffractive Diffuser

[0193]FIG. 14H shows features of a preferred embodiment providing evenfurther reduction of light intensity passing through the etalon. Thisembodiment is similar to the embodiment discussed above. The sample beamfrom mirror 182 (approximately 15 mm×3 mm) passes upward throughcondensing lens 400 and is then re-collimated by lens 402. The beam nowcolliminated and reduced in dimension to about 5 mm×1 mm passes throughetalon housing window 404 and then passes through a diffractivediffusing element 406 which in this case (for an ArF laser) is adiffractive diffusing element provided by Mems Optical, Inc. withoffices in Huntsville, Ala. The element is part number D023-193 whichconverts substantially all 193 nm light in any incoming collimated beamof any cross sectional configuration into a beam expanding in a firstdirection at 2° and in a second direction perpendicular to the firstdirection at 4° . Lens 410 then “focuses” the expanding beam onto arectangular pattern covering photodiode array 180 shown in FIG. 14. Theactive area of the photo diode array is about 0.5 mm wide and 25.6 mmlong and the spot pattern formed by lens 410 is about 15 mm×30 mm.Diffractive diffusing element thoroughly mixes the spacial components ofthe beam but maintains substantially all of the beam energy within the2° and 4° limits so that the light passing through the etalon can besubstantially reduced and efficiently utilized. The reader shouldrecognize that further reductions in beam energy passing through theetalon could be realized by reducing the spot pattern in the shortdimension of the photo diode array. However, further reductions to lessthan 15 mm will make optical alignment more difficult. Therefore, thedesigner should consider the spot pattern size to be a trade-off issue.

[0194] In another system designed for a KrF laser operating at about248.327 nm a similar design is provided with adjustments for wavelength.In this embodiment lens 400 has a focal length of about 50 mm. (The lensis Melles Griot Corporation part number OILQP001.) Collimating lens 402has a focal length of −20 mm (EVI Laser Corporation part numberPLCC-10.0-10.3-UV). The diffractive diffusing element 406 is MemsOptical Corporation part number DO23-248. In this embodiment and in theArF embodiment, the spacing between the two lenses can be properlypositioned with spacer 416. Applicants estimate that the energy of thebeam passing through the etalon with the laser operating at 2000 Hz isabout 10 mw and is not sufficient to cause significant thermal problemsin the etalon. In other preferred embodiments, the beam could be allowedto come to a focus between lenses 400 and 402. Appropriate lenses wouldin this case be chosen using well known optical techniques.

Feedback Control of Pulse Energy and Wavelength

[0195] Based on the measurement of pulse energy of each pulse asdescribed above, the pulse energy of subsequent pulses are controlled tomaintain desired pulse energies and also desired total integrated doseof a specified number of pulses all as described in U.S. Pat. No.6,005,879, Pulse Energy Control for Excimer Laser which is incorporatedby reference herein.

[0196] Wavelength of the laser may be controlled in a feedbackarrangement using measured values of wavelengths and techniques known inthe prior art such as those techniques described in U.S. Pat. No.5,978,394, Wavelength System for an Excimer Laser also incorporatedherein by reference. Applicants have recently developed techniques forwavelength tuning which utilize a piezoelectric driver to provideextremely fast movement of tuning mirror. Some of these techniques aredescribed in U.S. patent application Ser. No. 608,543, Bandwidth ControlTechnique for a Laser, filed Jun. 30, 2000 which is incorporated hereinby reference. FIGS. 16A and 16B are extracted from that application andshow the principal elements of this technique. A piezoelectric stack isused for very fast mirror adjustment and larger slower adjustments areprovided by a prior art stepper motor operating a lever arm. Thepiezoelectric stack adjusts the position of the fulcrum of the leverarm.

NEW LNP WITH COMBINATION PZT-STEPPER MOTOR DRIVEN TUNING MIRROR DetailDesign with Piezoelectric Drive

[0197]FIG. 16 is a block diagram showing features of the laser systemwhich are important for controlling the wavelength and pulse energy ofthe output laser beam. Shown are a line narrowing module 15K whichcontains a three prism beam expander, a tuning mirror 14 and a grating.Wavemeter 104 monitors the output beam wavelength and provides afeedback signal to LNP processor 106 which controls the position oftuning mirror 14 by operation of a stepper motor and a PZT stack asdescribed below. Operational wavelengths can be selected by lasercontroller 102. Pulse energy is also measured in wavemeter 104 whichprovides a signal used by controller 102 to control pulse energy in afeedback arrangement as described above. FIG. 16A is a block diagramshowing PZT stack 80, stepper motor 82, mirror 14 and mirror mount 86.

[0198]FIG. 16B1 is a drawing showing detail features of a preferredembodiment of the present invention. Large changes in the position ofmirror 14 are produced by stepper motor through a 26.5 to 1 lever arm84. In this case a diamond pad 81 at the end of piezoelectric drive 80is provided to contact spherical tooling ball at the fulcrum of leverarm 84. The contact between the top of lever arm 84 and mirror mount 86is provided with a cylindrical dowel pin on the lever arm and fourspherical ball bearings mounted (only two of which are shown) on themirror mount as shown at 85. Piezoelectric drive 80 is mounted on theLNP frame with piezoelectric mount 80A and the stepper motor is mountedto the frame with stepper motor mount 82A. Mirror 14 is mounted inmirror mount 86 with a three point mount using three aluminum spheres,only one of which are shown in FIG. 16B1. Three springs 14A apply thecompressive force to hold the mirror against the spheres.

[0199]FIG. 16B2 is a preferred embodiment slightly different from theone shown in FIG. 16B1. This embodiment includes a bellows 87 to isolatethe piezoelectric drive from the environment inside the LNP. Thisisolation prevents UV damage to the piezoelectric element and avoidpossible contamination caused by out-gassing from the piezoelectricmaterials.

Test Results

[0200]FIG. 16C shows actual test data from a laser fitted with the FIG.16B2 embodiment. The graph is a plot of the deviation from targetwavelength of the average of 30 pulse windows. The deviation is reducedfrom about 0.05 pm to about 0.005 pm.

[0201] This embodiment is a major speed up as compared to the steppermotor drive system described above but not quite fast enough forpulse-to-pulse adjustment. Earlier methods of mirror positioningrequired about 7 ms to move mirror 14, making pulse-to-pulse wavelengthcorrection at 2000 Hz out of the question. In that earlier technique, alever arm pivoted about a pivot axis to produce a 1 to 26.5 reduction inthe mirror movement compared to the stepper position movement. The priorart stepper has a total travel of ½ inch (12.7 mm) and 6000 steps sothat each step is a distance of about 2 microns. With the 1-26.5reduction, one step moves the mirror about 75 nm which typically changesthe wavelength of the laser wavelength about 0.1 pm. In the fast actingtechnique shown in FIG. 12A, a piezo stack 80 has been added at thepivot position of the lever arm. A preferred piezo stack is ModelP-840.10 supplied by Physik Instrumente GmbH with offices in Waldbronn,Germany.

[0202] This stack will produce linear adjustment of about 3.0 micronswith a drive voltage change of 20 volts. This range is equivalent toabout ±20 steps of the stepper motor.

[0203] The stack responds to a control signal within less than 1microsecond and the system can easily respond to updated signals at afrequency of 4000 Hz. In a preferred embodiment the control for eachpulse at 4000 Hz pulse rate is based not on the previous pulse but thepulse prior to the previous pulse to allow plenty of time for thewavelength calculation. However, this embodiment provides a factor of 7improvement over the prior art design with a 7 millisecond latency.Therefore, much faster feedback control can be provided. One preferredfeedback control algorithm is described in FIG. 16D. In this algorithmthe wavelength is measured for each pulse and an average wavelength forthe last four and last two pulses is calculated. If either of theaverages deviate from the target wavelength by less than 0.02 pm, noadjustment is made. If both deviate more than 0.02 pm from the target,an adjustment is made to the mirror assembly by piezoelectric stack 80to provide a wavelength correction. Which of the two averages is used isdetermined by how much time had elapsed since the last adjustment. Thepiezoelectric stack is maintained within its control range by steppingthe stepper motor as the stack approaches 30 and 70 percent of its range(or to provide more available range, 45 and 55 percent could be usedinstead of the 30 and 70 percent range values). Since the stepper motorrequires about 7 ms to complete a step, the algorithm may make severalpiezo adjustments during a stepper motor step.

Pretuning and Active Tuning

[0204] The embodiments described above can be used for purposes otherthan chirp corrections. In some cases the operator of a integratedcircuit lithography machine may desire to change wavelength on apredetermined basis. In other words the target wavelength λ_(T) may notbe a fixed wavelength but could be changed as often as desired eitherfollowing a predetermined pattern or as the result of a continuously orperiodically updating learning algorithm using early historicalwavelength data or other parameters.

Adaptive Feedforward

[0205] Preferred embodiments of the present invention includesfeedforward algorithms. These algorithms can be coded by the laseroperator based on known burst operation patterns. Alternatively, thisalgorithm can be adaptive so that the laser control detects burstpatterns such as those shown in the above charts and then revises thecontrol parameters to provide adjustment of mirror 14 in anticipation ofwavelength shifts in order to prevent or minimize the shifts. Theadaptive feedforward technique involves building a model of the chirp ata given rep rate in software, from data from one or more previous burstsand using the PZT stack to invert the effect of the chirp. To properlydesign the chirp inversion, two pieces of information are needed: (1)the pulse response of the PZT stack, and (2) the shape of the chirp. Foreach repetition rate, deconvolution of the chirp waveform by the pulseresponse of the PZT stack will yield a sequence of pulses, which, whenapplied to the PZT stack (with appropriate sign), will cancel the chirp.This computation can be done off line through a survey of behavior at aset of repetition rates. The data sequences could be saved to tablesindexed by pulse number and repetition rate. This table could bereferred to during operation to pick the appropriate waveform data to beused in adaptive feedforward inversion. It is also possible, and in factmay be preferable, to obtain the chirp shape model in almost real-timeusing a few bursts of data at the start of operation each time therepetition rate is changed. The chirp shape model, and possibly the PZTpulse response model as well, could then be updated (e.g. adapted) everyN-bursts based on accumulated measured error between model and data. Apreferred algorithm is described in FIG. 16E.

[0206] The chirp at the beginning of bursts of pulses can be controlledusing the algorithm described in FIG. 16E. The letter k refers to thepulse number in a burst. The burst is separated into two regions, a kregion and an l region. The k region is for pulse numbers less thank_(th) (defining a time period long enough to encompass the chirp).Separate proportional constant P_(k), integral constant I_(k) andintegral sum of the line center error ΣLCE_(k) are used for each pulsenumber. The PZT voltage for the corresponding pulse number in the kregion in the next burst is determined by these constants and sums.After the kth pulse, a traditional proportional integral routinecontrols the PZT voltage. The voltage for next pulse in the burst willbe the current voltage plus P*LCE+I*ΣLCE. A flow diagram explaining themajor steps in this algorithm is provided in FIG. 16E.

HIGH DUTY CYCLE LNP

[0207] It is known to purge line narrowing packages; however, the priorart teaches keeping the purge flow from flowing directly on the gratingface so that purge flow is typically provided through a port located atpositions such as behind the face of the grating. Applicants havediscovered, however, that at very high repetition rates a layer of hotgas (nitrogen) develops on the face of the grating distorting thewavelength. This distortion can be corrected at least in part by theactive wavelength control discussed above. Another approach is to purgethe face of the grating as shown in FIG. 17. In FIG. 17, small holes (1mm on ¼ inch spacings) in the top of 10-inch long ⅜ inch diameter purgetube 61 provides the purge flow. The purge gas can be nitrogen from apure nitrogen supply as described in a following section. Also, heliumcan be used as the purge gas and it can be more effective at removingheat from the LNP components. Other techniques are shown in FIGS. 17A,17B and 17C.

ULTRA PURE NITROGEN PURGE SYSTEM

[0208] This first embodiment of the present invention includes anultra-pure N₂ purge system which provides greatly improved performanceand substantially increases component lifetime.

[0209]FIG. 19 is a block diagram showing important features of a firstpreferred embodiment the present invention. Five excimer lasercomponents which are purged by nitrogen gas in this embodiment of thepresent system are LNP 2P, high voltage components 4P mounted on laserchamber 6P, high voltage cable 8P connecting the high voltage components4P with upstream pulse power components 10P, output coupler 12P andwavemeter 14P. Each of the components 2P, 4P, 8P, 12P, and 14P arecontained in sealed containers or chambers each having only two ports anN₂ inlet port and an N₂ outlet port. An N₂ source 16P which typically isa large N₂ tank (typically maintained at liquid nitrogen temperatures)at a integrated circuit fabrication plant but may be a relatively smallbottle of N₂. N₂ source gas exits N₂ source 16P, passes into N₂ purgemodule 17P and through N₂ filter 18P to distribution panel 20Pcontaining flow control valves for controlling the N₂ flow to the purgedcomponents. With respect to each component the purge flow is directedback to the module 17P to a flow monitor unit 22P where the flowreturning from each of the purge units is monitored and in case the flowmonitored is less than a predetermined value an alarm (not shown) isactivated.

[0210]FIG. 19A is a line diagram showing specific components of thispreferred embodiment including some additional N₂ features notspecifically related to the purge features of the present invention.

N₂ Filter

[0211] An important feature of the present invention is the inclusion ofN₂ filter 18. In the past, makers of excimer lasers for integratedcircuit lithography have believed that a filter for N₂ purge gas was notnecessary since N₂ gas specification for commercially available N₂ isalmost always good enough so that gas meeting specifications is cleanenough. Applicants have discovered, however, that occasionally thesource gas may be out of specification or the N₂ lines leading to thepurge system may contain contamination. Also lines can becomecontaminated during maintenance or operation procedures. Applicants havedetermined that the cost of the filter is very good insurance against aneven low probability of contamination caused damage.

[0212] A preferred N₂ filter is Model 500K Inert Gas Purifier availablefrom Aeronex, Inc. with offices in San Diego, Calif. This filter removesH₂O, O₂, CO, CO₂, H₂ and non-methane hydrocarbons tosub-parts-per-billion levels. It removes 99.9999999 percent of allparticulate 0.003 microns or larger.

Flow Monitors

[0213] A flow monitor in unit 22 is provided for each of the five purgedcomponents. These are commercially available units having an alarmfeature for low flow.

Piping

[0214] Preferably all piping is comprised of stainless steel (316SST)with electro polished interior. Certain types of plastic tubing,comprised of PFA 400 or ultra-high purity Teflon, may be used.

Recirculation

[0215] A portion or all of the purge gas could be recirculated as shownin FIG. 19B. In this case, a blower and a water cooled heat exchanger isadded to the purge module. For example, purge flow from the opticalcomponents could be recirculated and purge flow from the electricalcomponents could be exhausted or a portion of the combined flow could beexhausted.

Advantages of the System

[0216] The system described herein represents a major improvement inlong term excimer laser performance especially for ArF and F₂ lasers.Contamination problems are basically eliminated which has resulted insubstantial increases in component lifetimes and beam quality. Inaddition, since leakage has been eliminated except through outlet portsthe flow can be controlled to desired values which has the effect ofreducing N₂ requirements by about 50 percent.

SEALED SHUTTER UNIT WITH POWER METER

[0217] This first preferred embodiment includes a sealed shutter unit500 with a built in power meter as shown in FIGS. 20, 20A and 20B. Withthis important improvement, the shutter has two functions, first, as ashutter to block the laser beam and, second, as a full beam power meterfor monitoring beam power whenever a measurement is needed.

[0218]FIG. 20 is a top view showing the main components of the shutterunit. These are shutter 502, beam dump 504 and power meter 506. The pathof the laser output beam with the shutter in the closed position isshown at 510 in FIG. 20. The path with the beam open is shown at 512.The shutter active surface of beam stop element 516 is at 45° with thedirection of the beam exiting the chamber and when the shutter is closedthe beam is both absorbed in the shutter surface and reflected to beamdump 504. Both the beam dump active surface and the shutter activesurface stop element 516 is mounted on flexible spring steel arm 518.The shutter is opened by applying a current to coil 514 as shown in FIG.20B which pulls flexible arm 518 and beam stop element 516 to the coilremoving beam stop element 516 from the path of the output laser beam.The shutter is closed by stopping the current flow through coil 514which permits permanent magnets 520 to pull beam stop element 516 andflexible arm 518 back into the close position. In a preferred embodimentthe current flow is carefully tailored to produce an easy transmit ofthe element and arm between the open and close positions.

[0219] Power meter 506 is operated in a similar fashion to placepyroelectric photo detector in the path of the output laser beam asshown in FIGS. 20 and 20A. In this case, coil 520 and magnets 522 pulldetector unit 524 and its flexible arm 526 into and out of the beam pathfor output power measurements. This power meter can operate with theshutter open and with the shutter closed. Current to the coil is as withthe shutter controlled to provide easy transit of unit 524 into and outof the beam path.

BEAM SEAL SYSTEMS First Chamber Exit Seal Unit

[0220] Ultraviolet light in the spectral range of the ArF laser candamage sensitive optical components in the presence of oxygen or a widevariety of other chemical components. Also, oxygen is a significantabsorber of the ArF laser beam. For these reasons special provisions aremade to isolate the laser beam line between laser modules fromatmospheric air while permitting quick and easy replacement of themodules. Since a substantial amount of vibrational forces are generatedin the laser chamber the beam seal components between the chamber andthe LNP and between chamber, and the output coupler are preferablydesigned to minimize the transfer of vibrational forces to the opticalcomponents in the LNP and output coupler unit. Two special beam sealingbellows unit are described below which can be used on both the LNP sideof the chamber and the output coupler side of the chamber. These sealunits:

[0221] 1) contain no elastomers

[0222] 2) provide vibration isolation for the LNP and the OC fromchamber vibration

[0223] 3) provide beam train isolation from atmospheric gases

[0224] 4) permit unrestricted replacement of the chamber withoutdisturbance of the LNP and the output coupler.

[0225] The laser chamber weights more than 200 pounds and is typicallyrolled into position in the laser cabinet on little wheels as shown inFIGS. 22A, 22B, 22C and 22D. In one embodiment as shown in FIG. 22E, theseal unit (between the chamber window block 156A and the LNP front plate178) comprises a metal bellows 158, bellows flange 159 and protectorbracket 160. FIGS. 22E, 22F and 22G show the bellows unit beingcompressed as the chamber is rolled into position in the directionindicated by arrow 22F. An exploded view showing this bellows seal unitis shown in FIG. 22H a back side flange of bellows 158 is bolted to LNPfront plate 157A. Note that protection bracket 160 is attached to plate178 at threaded location 15A with bolts that allow side-ways slipping ofbracket 160 as it is compressed. The purge gas enters the LNP at a portat 16A and flows up across the grating face as described above byreference to FIG. 17.

[0226] Thus, purge gas flows into the bellows region through LNPaperture 18A and diffusion slots 19A then into window block 156A andfrom there to purge module 17 as shown in FIG. 19 where the flow can bemonitored and then exhausted. This bellows provides a very good sealreducing the oxygen content in the LNP to less than 100 parts permillion while at the same time permitting easy chamber replacement andminimizing vibration transfer from the chamber to the LNP. The seal isnot 100% effective since it relies on a surface to surface contactbetween the bellows flange 159 and the facing surface of window block156A with only moderate contact force of about 2 pounds applied by thebellows spring force.

Second Chamber Exit Seal Unit

[0227] An alternative seal unit is described in FIGS. 22I, 22J, 22K and22L. This bellows unit as shown in FIG. 22I is sealed with metal sealsat both interfaces with the chamber window block and the LNP frontplate. This unit includes two bellows 26A and 26B separated by metalcylinder 24A. Flange 20 is bolted to the chamber window block 56A andsealed with a metal “C” seal positioned at 28A as shown in FIG. 22Iprior to insertion of the chamber unit into the laser cabinet. Thechamber is then rolled into the laser cabinet as indicated in FIGS.22A-D. When the chamber is in position flange 22A is clamped to LNPfront plate 157A with V-clamp unit 31A and sealed with a metal “C” seallocated at 30A as shown in FIG. 22I.

[0228] The V-clamp unit which is a part of LNP module is shown in FIG.22I and 22K. The V-clamp works as follows. The V-clamp shown in FIG. 22Iis mounted on LNP frame 178 with bolts at 50A. Torsion spring 52A holdsthe front edge 47A of yoke-like lever 46A about 1 cm off the surface ofLNP frame 178 (not shown here). As chamber 156 is rolled into position,clampable flange 22A passes very close to the surface of LNP frame 178until the outer edge of clampable flange 22A is positioned betweenyoke-like lever 46A and the surface of LNP frame 178.

[0229] When chamber 156 is in its proper position between LNP 120 andoutput coupler 130, clampable flange 22A is clamped into position byrotating activation handle 44A 90° to 180° (into the page on the FIG.22I drawing). Cams 38A being offset from the axis of shaft 40A applies aforce out of the page (in the FIG. 22J drawings) against the undersideof extensions 45A of yoke-like lever 46A which forces the 45B portion oflever 46 downward clamping clampable flange 22A into position. A metal“C” seal in slot 30A is compressed by the clamping force providing anair-tight seal between the bellows structure 19A and LNP frame 178. FIG.22K shows the operation of the V-clamp unit.

[0230]FIG. 22L shows the bellows unit in place sealing the chamber-LNPinterface. This is a cross-section top view. Shown on the drawing aremetal “C” seals at 54A and 56A, chamber window block 156A, purge venthole 58A, chamber window 61A with seal 60A. Arrow 62 shows where theouter edge 22A of clampable flange 22 is clamped against LNP frame 178by yoke-like lever 46A (not shown here).

[0231] This bellows unit FIGS. 22I-L provides a much tighter sealbetween the chamber and the LNP and between the chamber and the outputcoupler than the one shown in FIGS. 22E-H; however, it is somewhat moreexpensive and the transfer of vibrational components through it may besomewhat greater.

[0232] For both units, although the detailed description referred to theLNP-chamber interface, the same unit and technique is preferably used toseal the chamber-output coupler interface.

Improved WaveMeter Purge

[0233] In this preferred embodiment a special N₂ purge technique is usedto provide extra purging of the high ultraviolet flux portions of thewavemeter as well as the output coupler and the chamber output windowblock. This technique is shown in FIG. 22M. As explained above the laseroutput beam intersects partially reflecting mirror 170 (see also FIG.14) which passes 95% of the energy in the beam as an output beam. About4% of the reflected beam is reflected from mirror 171 to energy detector172 where the pulse energy is measured. (The other part of the reflectedbeam passes through mirror 171 as shown at 61A and goes to othermonitors in the wave meter.) At 4,000 Hz this 5% of the output energyrepresents a lot of UV light so special care has been taken to assurethat the gas in the path of this portion of the beam is very clean andpure. To do this the wavemeter is modified to seal the region betweenthe upstream side of mirror 170, the upstream side of mirror 171 and thefront side of the window of detector 172 from the rest of the wavemeter.And a special purge flow to and from this region is provided as shown at62A. The remainder of the wavemeter is purged by a second purge flowshown at 64A.

[0234] The purge flow 62A is confined in the wavemeter by seals atmirrors 170, 171 and the 172 detector window. The purge flow exits thisregion along the laser output beam path through a bellows region 6A backto the output coupler module 68A to purge it. The flow then flowsthrough bellows unit 70A and into window block 72A, out through an exitport in the window block and an exit port in bellows unit 70A then backthrough a tube to N₂ purge module 17 as shown at 74A and in FIG. 19. Thedownstream side of window 170 is purged with purge flow from shuttermodule 5K. The purge flow may be from module 17 as shown in FIG. 19 orin some cases window 76A is removed and the output of shutter module isopenly connected with a purged customer beam line in which case the exitpurge line at 78A may be directed to a customer purge return system orexhausted to atmosphere.

[0235] Various modifications may be made to the present inventionwithout altering its scope. Those skilled in the art will recognize manyother possible variations. For example, active feedback control ofbandwidth can be provided by adjusting the curvature of the linenarrowing grating using a motor driver to adjust the bending mechanismshown in FIG. 22A. Or much faster control of bandwidth could be providedby using piezoelectric devices to control the curvature of the grating.Other heat exchanger designs should be obvious modifications to the oneconfiguration shown herein. For example, all four units could becombined into a single unit. There could be significant advantages tousing much larger fins on the heat exchanger to moderate the effects ofrapid changes in gas temperature which occurs as a result of burst modeoperation of the laser. The reader should understand that at extremelyhigh pulse rates the feedback control on pulse energy does notnecessarily have to be fast enough to control the pulse energy of aparticular pulse using the immediately preceding pulse. For example,control techniques could be provided where measured pulse energy for aparticular pulse is used in the control of the second or third followingpulse. Many variations and modifications in the algorithm for convertingwavemeter etalon and grating data to wavelength values are possible. Forexample, Applicants have discovered that a very minor error results froma focusing error in the etalon optical system which causes the measuredline width to be larger than it actually is. The error increasesslightly as the diameter of the etalon fringe being measured getslarger. This can be corrected by scanning the laser and a range ofwavelengths and watch for step changes as the measured fringes leave thewindows. A correction factor can then be determined based on theposition of the measured fringes within the windows. Accordingly, theabove disclosure is not intended to be limiting and the scope of theinvention should be determined by the appended claims and their legalequivalents.

We claim:
 1. An extreme repetition rate gas discharge laser capable ofoperating at pulse repetition rates in the range of 4,000 pulses persecond, said laser comprising: A) a laser chamber containing a laser gasand having two elongated electrodes, defining a discharge region andhaving a gas flow path with a gradually increasing cross sectiondownstream of said electrodes to permit recovery of a large percentageof static pressure drop occurring in the discharge region, B) atangential type fan for producing sufficient gas velocities of saidlaser gas in said discharge region to clear from said discharge region,following each pulse, substantially all discharge produced ions prior toa next pulse when operating at a repetition rate in the range of 4,000pulses per second or greater, C) a heat exchanger system capable ofremoving at least 16 kw of heat energy from said laser gas, D) a pulsepower system configured to provide electrical pulses to said electrodessufficient to produce laser pulses at rates of about 4,000 pulses persecond with precisely controlled pulse energies in the range of about 5mJ, and E) a laser beam measurement and control system capable ofmeasuring pulse energy energy wavelength and bandwidth of energy pulsesor substantially every pulse with feedback control of pulse energy andwavelength.
 2. A laser as in claim 1 wherein said gradually increasingcross section down stream of said discharge region increases at about 20degrees.
 3. A laser as in claim 2 and wherein said chamber alsocomprises a vane structure upstream of said discharge region fornormalizing gas velocity upstream of said discharge region.
 4. A laseras in claim 1 wherein said fan comprises a shaft driven by two brushlessDC motors.
 5. A laser as in claim 4 wherein said motors are water cooledmotors.
 6. A laser as in claim 4 wherein each of said motors comprise astator and each of said motors comprise a magnetic rotor contained in apressure cup separating a said stator from said laser gas.
 7. A laser asin claim 4 wherein said tangential fan comprise a blade structuremachined from said aluminum stock.
 8. A laser as in claim 7 wherein saidblade structure has an outside diameter of about five inches.
 9. A laseras in claim 4 wherein said motors are sensorless motors and furthercomprising a master motor controller for controlling one of said motorsand a slave motor controller for controlling the other motor.
 10. Alaser as in claim 1 wherein said finned heat exchanger system is watercooled.
 11. A laser as in claim 10 wherein said heat exchanger systemcomprises at least four separate water cooled heat exchangers.
 12. Alaser as in claim 10 wherein heat exchanger system comprises at leastone heat exchanger having a tubular water flow passage wherein at leastone turbulator is located in said path.
 13. A laser as in claim 11wherein each of said four heat exchangers comprise a tubular water flowpassage containing a turbulator.
 14. A laser as in claim 1 wherein saidpulse power power system comprise water cooled electrical components.15. A laser as in claim 14 wherein at least one of said water cooledcomponents is a component operated at high voltages in excess of 12,000volts.
 16. A laser as in claim 15 wherein said high voltage is isolatedfrom ground using an inductor through which cooling water flows.
 17. Alaser as in claim 1 wherein said pulse power system comprises a resonantcharging system to charge a charging capacitor to a precisely controlledvoltage.
 18. A laser as in claim 17 wherein said resonance chargingsystem comprises a De-Qing circuit.
 19. A laser as in claim 17 whereinsaid resonance charging system comprises a bleed circuit.
 20. A laser asin claim 17 wherein said resonant charging system comprises a De-Qingcircuit and a bleed circuit.
 21. A laser as in claim 1 wherein saidpulse power system comprises a charging system comprised of at leastthree power supplies arranged in parallel.
 22. A laser as in claim 1wherein said laser beam measurement and control system comprises anetalon unit, a photo diode array, a programmable logic device, andoptics to focus laser light from said etalon unit on to said photo diodearray wherein said programmable logic device is programmed to analyzedata from said photodiode array to determine locations on said photodiode array of etalon fringes.
 23. A laser as in claim 22 wherein saidmeasurement an control system also comprises a microprocessor programmedto calculate wavelength and bandwidth from fringe data located by saidprogrammable logic device.
 24. A laser as in claim 22 wherein saidprogrammable logic device is programmed with an algorithm forcalculating wavelength and bandwidth based on measurement of saidfringes.
 25. A laser as in claim 24 wherein said programmable logicdevice is configured to make calculations of wavelength and bandwidthfaster than {fraction (1/4,000)} of a second.
 26. A laser as in claim 22wherein said etalon unit comprises a defractive diffusing element.
 27. Alaser as in claim 1 and further comprising a line narrowing unitcomprising a tuning mirror driven at least in part by a PZT drive.
 28. Alaser as in claim 27 wherein said tuning mirror is also driven in partby a stepper motor.
 29. A laser as in claim 27 and further comprising apretuning means.
 30. A laser as in claim 27 and further comprising anactive tuning means comprising a learning algorithm.
 31. A laser as inclaim 27 and further comprising an adaptive feed forward algorithm. 32.A laser as in claim 27 wherein said line narrowing unit comprises agrating defining a grating face and a purge means for forcing purge gasadjacent to said grating face.
 33. A laser as in claim 32 wherein saidpurge gas is nitrogen.
 34. A laser as in claim 32 wherein said purge gasis helium.
 35. A laser as in claim 1 and further comprising a nitrogenpurge system comprising a nitrogen purge system comprising a nitrogenfilter.
 36. A laser as in claim 1 and further comprising a nitrogencomprising a purge module comprising flow monitors said laser alsocomprising purge exhaust tubes for transporting exhaust purge gas fromsaid laser.
 37. A laser as in claim 1 and further comprising a shutterunit comprising an electrically operated shutter and a power meter whichcan be positioned in a laser output beam path with a command signal. 38.A laser as in claim 27 and further comprising a beam seal systemproviding a first beam seal between a first window of said chamber andline narrowing unit and a second beam seal between a second window ofsaid chamber and an output coupler unit, each of said beam sealscomprising a metal bellows.
 39. A laser as in claim 38 wherein each ofsaid first and second beam seals are configured to permit easyreplacement of said laser chamber.
 40. A laser as in claim 38 whereineach of said beam seals contain no elastomer, provide vibrationisolation from said chamber, provide beam train isolation fromatmospheric gases and permit unrestricted replacement of said laserchamber without disturbance of said LNP or said output coupler unit. 41.A laser as in claim 1 wherein said measurement and control systemcomprises a primary beam splitter for splitting off a small percentageof output pulses from said laser, a second beam splitter for directing aportion of said small percentage to said pulse energy detector and ameans isolating a volume bounded said primary beam splitter, saidsecondary beam splitter and a window of said pulse energy detector fromother portions of said measurement and control system to define anisolated region.
 42. A laser as in claim 41 and further comprising apurge means for purging said isolated region with a purge gas.
 43. Alaser as in claim 42 wherein said laser further comprises an outputcoupler unit and an output window unit said purge means being configuredso that exhaust from said isolated region also purges said outputcoupler unit and said output window unit.